1. Field of the Invention
The present invention relates to a zoom lens system and a camera having the same.
2. Description of the Related Art
In recent years, digital still cameras having a solid-state image pickup element have become increasingly sophisticated and small. Such cameras require, as a photographing optical system, a zoom lens that is compact yet has a high zoom ratio.
There are known, as zoom lenses meeting such requirements, four-unit zoom lenses each including four lens units consisting of, in order from the object side to the image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of positive refractive power (Japanese Patent Laid-Open No. 7-270684 corresponding to U.S. Pat. No. 5,963,378, Japanese Patent Laid-Open No. 11-305124 corresponding to U.S. Pat. Nos. 6,166,864 and 6,751,028). In this four-unit zoom lens, the second lens unit is moved to vary magnification, and the fourth lens unit is moved to compensate the image plane variation accompanying the magnification variation and to perform focusing.
In general, it is effective to retract lens units for saving space to store a camera not in use. However, the zoom lenses disclosed in Japanese Patent Laid-Open Nos. 7-270684 and 11-305124, in which the second lens unit has almost all magnification variation functions, are unfit for the retractable structure because the first lens unit and the second lens unit are too sensitive to decentration.
There are also known zoom lenses suitable for the retraction and each including four lens units consisting of, in order from the object side to the image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of positive refractive power (Japanese Patent Laid-Open No. 10-62687 corresponding to U.S. Pat. No. 6,016,228, Japanese Patent Laid-Open No. 2001-194586 corresponding to U.S. Pat. No. 6,456,441, Japanese Patent Laid-Open No. 2003-315676 corresponding to U.S. Pat. No. 6,975,461, Japanese Patent Laid-Open No. 2005-24844 corresponding to U.S. Pat. No. 6,972,909, Japanese Patent Laid-Open No. 2006-171655 corresponding to U.S. Pat. No. 7,206,137, Japanese Patent Laid-Open No. 2006-184413 corresponding to U.S. Patent Appl. Pub. No. 2006/0140606, and Japanese Patent Laid-Open No. 2006-133632 corresponding to U.S. Pat. No. 7,190,529).
In each of the zoom lenses of Japanese Patent Laid-Open Nos. 10-62687 and 2001-194586, the first lens unit is a single lens, the distances between the lens units are changed to perform zooming, and the fourth lens unit is moved to perform focusing. Therefore, these zoom lenses are simple and suitable for the retractable structure and have a zoom ratio of about 3.
Japanese Patent Laid-Open No. 2003-315676 discloses a zoom lens that performs zooming by moving first, second, third, and fourth lens units and that has a zoom ratio of about 5.
The zoom lens of Japanese Patent Laid-Open No. 2005-24844 is effectively downsized by appropriately selecting lens materials of the second lens unit.
Japanese Patent Laid-Open Nos. 2006-171655, 2006-184413, and 2006-133632 disclose zoom lenses that perform zooming by moving first, second, third, and fourth lens units and that have a high zoom ratio of 10 or more.
In general, to reduce the size of a photographing optical system, the refractive power of each lens unit constituting the photographing optical system is increased, and the number of lenses is reduced. However, a photographing optical system whose refractive power is simply increased is not as compact as expected because the lens thickness increases with the increase in refractive power. In addition, compensation of aberrations becomes difficult.
When lens units of a camera not in use are retracted, errors such as inclination of lenses and lens units are inevitably large due to the mechanical structure. If the lenses and the lens units are sensitive, the optical performance deteriorates and image shake occurs during zooming. Therefore, in a photographing optical system, the sensitivity of the lens and the lens units should be minimized.